TV Freeview picture breaking up, New 4G transmitter and Aeroplanes

[Jim Lesurf]

Erm... within a set of limits and assumptions.

Jim

[Jim Lesurf]

Which would be a very different frequency to DVB-T/T2 and coming from a very different azimuth into a more directional RX antenna.

Jim

[Bill Wright]

That's another one. "The picture goes off when the wind blows from the north." Often true when the aerial's loose!

When I was doing the 'wind turbine amelioration' work there was one woman who had made a really good detailed diary who showed the close correlation between the behaviour of the turbines (which were clearly visible from her living room) and her TV reception. When the turbines were turning fast her satellite reception would break up. Therefore, she said, the wind turbine company must pay to fix her reception, and if they couldn't do it they would jolly well have to pay for her to move house to the bungalow that she had her eye on in the next village.

Her dish had been fixed (for no good reason) at the top of a ten foot aerial mast, and when the wind blew (and the turbines whizzed round like billio) the mast deflected and the dish went momentarily off beam. Thus we must always remember that correlation is not causation.

Bill

[john west]

Many thanks to all. I followed the suggestion to recheck the Aerial lead connection going into the Humax digi-box after remembering the visiting at800 engineer pulled it apart and seemed to re-assemble it a bit too quickly for my liking.

After re-doing this connection very carefully it now seems things are a bit better this morning. (Although mild weather and clear skies might be connected?).

Is it possible that the 4G phone companies although giving us a ' new filter' might be degrading our freeview reception in ways they are not very willing to talk about?

Our post code is NW7 1NE and the Bigger Aeroplanes which are at a higher altitude and pass between our house and the Crystal Palace Transmitter are now *not* breaking up our freeview reception as before.

But the smaller millionaire type planes like the 'Lear Jets' which are traveling at a lower altitude and which are traveling from east to west quite close to our house (presumably going to Heathrow and are at a lower altitude than the big planes) are *still* breaking up our reception for a few seconds 'every-time' one passes over.

Given that the much of the consensus in this group seems to be saying this kind of plane interference is very unlikely, should i have myself and my whole family enter the local Whittington Psychiatric Unit..... :)

Seriously though we are not imagining this, grateful for any further thoughts. Thanks.

[Dave Farrance]

Hmm, yes. I recall reading that Doppler effects are especially problematic for OFDM modulated signals, and is a limiting factor on the use of OFDM broadcasts to high-speed road vehicles.

Presumably, given the huge number of subcarriers of DVB-T2 (do we use

32k in the UK?) and the speed of aircraft, then it would no longer be a simple case of multi-path interference suppressing a few carriers, but would be significantly destructive interference.

[Mark Carver]

T1 uses 8k, T2 uses 32k. Be interesting (in view of the new info the OP has just posted) whether the T1 and T2 muxes are affected or just T2 ?

[Mark Carver]

Is your TV able to receive HD channels, if so, are these channels the ones that break up, or just the SD channels, or both ?

[Bill Wright]

No, you've not read what I put carefully enough. 'Pretty well owt' referred to the phenomena people blame for their poor reception, not to real causes of same.

He said they were so high he couldn't read their markings. In any case a 'few thousand feet' would be enough for everything I said.

See above.

See above.

Again, you've not read what I put carefully enough. Or you don't understand why I carefully used the word 'generated'.

There are many other reasons why a long period of good reception can end. Many of them are far from obvious.

An interesting assertion. How many times have such people as Charles Hope (ex BBC) heard people blame 'the transmitter'? Let's take your words above, "The picture was fine for a couple of years, then for a few months it was awful, and now it's fine again." Without thinking too deeply, I can remember all of the following occurrences:

1. A kite attached a bit of itself to the aerial. Eventually this blew off.
2. Yes it really was the transmitter! Or actually it was one in another town on the same channel which had gone ever so slightly off tune, and thus was putting coarse horizontal lines across the picture.
3. A bloke moved in at the top of the street and parked his big van in the back alley every night, and it just happened to obstruct the signal of someone at the bottom the street. After a while he was promoted so he got a car, or maybe he died, or moved away, I dunno. But the van disappeared.
4. The aerial was slightly loose and swung slightly off beam, then back again.
5. Trees. Over and over again I've seen this sort of thing caused by trees. No rhyme or reason quite often. They don't need to be in the signal path.
6. A connection behind the TV was disturbed, then disturbed again a few weeks later.
7. Ditto under the carpet.
8. Ditto in the loft.
9. Ditto on the roof.

Bill

[Bill Wright]

Yes I'd think a plane really close would have much the same effect as a wind turbine similarly close. It's interesting to watch the effect on an analyser. A great 'orrible notch of infinite depth moves across the mux, taking a good proportion of the carriers down far enough that the AGC can't cope and/or the carrier's s/n ration is inadequate.

Bill

[Bill Wright]

That of course is screening not reflections.

Bill

[Dave Farrance]

Thanks. 32k it is then.

The UHF channel bandwidth is 8MHz, so each subcarrier is just 250Hz.

Crystal Palace PSB3 is on 545800000Hz, so shifting it by just half the above to 545800125Hz would be destructive.

Doppler shift: f' = f * sqrt((1 + v/c) / 1 -v/c))

I find that I can rearrange that to:

v = c * ((g - 1)/(g + 1)) where g = (f'/f)^2

Which gives me a speed of 68.6 m/s or 153.6 mph, which *is* aircraft landing speed, close enough.

[Mark Carver]

Yes, although the reflected signal would have to be of significant strength relative to the wanted, to have an effect. The aerial's peak gain will be in its horizontal plane (no pun ;-)) and in the direction of the transmitter. As stated it's low planes that seem to be causing the problem, possibly flying in or away on the same bearing as the transmitter ?

[Jim Lesurf]

In addition, a high dopper velocity may shift the reflections by a subcarrier interval or more. So overlapping the signals from different subcarriers. Combine that with all the brisk changes in phase and amplitude and you could get a mess that the RX can't unscramble.

However, as before, I've no idea if this *is* the cause in this case. Just that I wouldn't reject the possibility at first suggestion.

Jim

[Jim Lesurf]

It is possible that you'd benefit from a 'better' filter. Depends on the local circumstances.

Here, I'd done some checking in advance so anticipated problems. And they duly appeared when the local 4G base station went live. But here planes didn't come into it. (Despite there also being a RAF base in much the same direction at the time! 8-] ) So I wasn't bothered by Biggles and his chums.

To continue to get the full set of DVB multiplexes I needed one of the 'Ch60' 4G filters that have a tighter spec than the general purpose ones. (Translation, bigger and more expensive.) This is because we still get one TV transmission at the top of the band. i.e. as close in frequency as now possible to the 4G. And the base station is close, and in much the same direction as the TV transmitter. Fortunately, I'd talked to at800 in advance and they sent me a filter that did the job.

So it depends on what the levels, etc, are where your TV antenna is picking up things.

If none of your TV transmissions are in the top channels you might find that *two* filters in series help. But may not. The problem here is that if the planes *are* mucking up your TV reception then that problem is a severe one, so you may find that only a tiny amount of added interference tips it 'over the cliff' at times. And unless the filters are good, they can also lose some of the wanted signal, so simply throwing 'more filters' at it isn't a panacea.

But note the series of "ifs", etc. The way to find out would be to experiment. You can buy filters from places like CPC if at800 won't help further.

Jim

[Jim Lesurf]

Someone can now tell us/me what channels CP is broadcasting so we can check if they are near the top of the band. Since I live far from CP I don't keep such details in my head. :-)

Jim

[Dave Farrance]

I wondered what the strength of the interfering signal would have to be, so after some searching, I find that DVB-T2 seems to be more vulnerable that DVB-T for this too. The interfering signal has to be at least 27dB down or it will cause trouble (compared with 20dB for DVB-T, IIRC).

Table 6 here gives the failure point for co-channel interference (which would cause the same sort of problems as frequency-shifted signals).

So it seems that the Freeview HD channels are extra-vulnerable for more than one reason compared to Freeview SD.

[Dave Farrance]

We've continued discussing the issue, and it does seem that there is reason to think that Freeview HD (channels 101 ,102 etc) would be significantly more susceptible to aircraft interference than Freeview SD (channels 1, 2 etc). Can you compare that?

[The Natural Philosopher]

[Java Jive]

BTW, I d> >

[tony sayer]

You forgot water in the feeder cable thats now dried out but filled up again?.